As a catheter equipped with a dilatation element for dilating stenotic sections in blood vessels, so called Simpson-Robert type catheter disclosed in the specification of U.S. Pat. No. 4,323,071 is used.
The stenotic lesions in blood vessels which could be treated by the early models of this type of catheter were limited to those in the proximity of the coronary arteries anatomically, of about 15 to 20 mm in length, localized in a single branch, and not calcificated. But improved in order to expand the treatable lesions, later models have become able to treat more serious stenotic lesions such as sub-total obliteration and at more peripheral-side vessels.
The applicant of this application has previously proposed another catheter which comprises an inner tube, an outer tube disposed coaxial in the inner tube, and a dilatation element attached to the inner and outer tubes, as described in International Publication No. W088/6464.
In the Simpson-Robert type catheter described in U.S. Pat. No. 4,323,071 and the one disclosed in the International Publication No. W088/6464, the inner and outer tubes are fixed at their proximal ends, while the rear end of a dilatation element is attached to the distal end of the outer tube and the front end to the distal end of the inner tube. The outer and inner tubes are thus connected by the intermediary of the dilatation element at their distal ends.
The coaxial-type catheters as disclosed in U.S. Pat. No. 4,323,071 and in the International Publication No. W088/6464 are inserted into vessels with the dilatation element folded up around the inner tube. While being advanced in vessels, a small displacement of the relative position of the outer and inner tubes can occur due to the frictional resistance exerted over the dilatation element and the force from the inner wall of blood vessels against the inner tube when the tip of the inner tube is in contact with the vessel wall. This displacement between the outer and inner tubes pulls or pushes the ends o the folded dilatation element. This force can cause the deformation of the dilatation element and sometimes the dilatation element expands thicker in the outer diameter than in its completely folded state. In that case, manipulation of the catheter, particularly insertion of the dilatation element into stenotic sections becomes difficult.